Lecture 5: Lipids and Carbohydrates

Craig Kasper

Fish Nutrition

Part 1: Lipid Characteristics

� Lipid = a compound that is insoluble in water, but soluble in an organic solvent

(e.g., ether, benzene, acetone, chloroform)

� “lipid” is synonymous with “fat”, but

also includes phospholipids, sterols, etc.

� chemical structure: glycerol + fatty acids

Lipid Molecule

Nutritional Uses of Lipids

� We already know that lipids are concentrated sources of energy (9.45 kcal/g)

� other functions:

� 1) provide means whereby fat-soluble nutrients (e.g., sterols, vitamins) can be absorbed by the body

� 2) structural element of cell, subcellular components

� 3) components of hormones and precursors for prostaglandin synthesis

Lipid Classes

� simple: FA’s esterified with glycerol

� compound: same as simple, but with other compounds also attached

� phospholipids: fats containing phosphoric acid and nitrogen (lecithin)

� glycolipids: FA’s compounded with CHO, but no N

� derived lipids: substances from the above derived by hydrolysis

� sterols: large molecular wt. alcohols found in nature and combined w/FA’s (e.g., cholesterol)

Saturated vs. Unsaturated Fatty Acids

� saturated: the SFA’s of a lipid have no double bonds between carbons in chain

� polyunsaturated: more than one double bond in the chain

� most common polyunsaturated fats contain the polyunsaturated fatty acids (PUFAs) oleic, linoleic

and linolenic acid

� unsaturated fats have lower melting points

� stearic (SFA) melts at 70oC, oleic (PUFA) at 26oC

Fatty Acids Commonly Found in Lipids

Sat. Fatty Acids Formula Melting Point (oC)

Butyric C4H8O2 Liquid

Palmitic C16H22O2 63

Stearic C18H36O2 70

Unsat. Fatty Acids Formula Melting Point (oC)

Oleic C18H34O2 Liquid

Linoleic C18H32O2 Liquid

Linolenic C18H30O2 Liquid

Saturated vs. Unsaturated Fats

� saturated fats tightly packed, clog arteries as atherosclerosis

� because of double bonds, polyunsaturated fats do not pack well -- like building a wall with bricks (sat.) vs. irregular-shaped objects (unsat.)

� plant fats are much higher in PUFA’s than animal fats

Saturated vs. Unsaturated FA’s Plant vs. Animal Fat

corn soy tallow lard

Sat. FA’s

Myristic 3

Palmitic 7.0 8.5 27 32.2

Stearic 2.4 3.5 21 7.8

Unsat. FA’s

Oleic 45.6 17 40 48

Linoleic 45.0 54.4 2 11

Linolenic 7.1 0.5 0.6

Arachid.

Lipid Digestion/Absorption

� Fats serve a structural function in cells, as sources of energy, and insulation

� the poor water solubility of lipids presents a problem for digestion: substrates are not easily accessible to digestive enzymes

� even if hydrolyzed, the products tend to aggregate to larger complexes that make poor contact with the cell surface and aren’t easily absorbed

� to overcome these problems, changes in the physical state of lipids are connected to chemical changes during digestion and absorption

Lipid Digestion/Absorption

Five different phases:

� hydrolysis of triglycerides (TG) to free fatty acids (FFA) and monoacylglycerols

� solubilization of FFA and monoacylglycerols by

detergents (bile acids) and transportation from the intestinal lumen toward the cell surface

� uptake of FFA and monoacylglycerols into the cell and resynthesis to triglyceride

� packaging of TG’s into chylomicrons

� exocytosis of chylomicrons into lymph

Enzymes Involved in Digestion of Lipids

� lingual lipase: provides a stable interface with aqueous environment of stomach

� pancreatic lipase: major enzyme affecting triglyceride hydrolysis

� colipase: protein anchoring lipase to the lipid

� lipid esterase: secreted by pancreas, acts on cholestrol esters, activated by bile

� phospholipases: cleave phospholipids, activated by trypsin

What about Bile???

� These are biological detergents synthesized by the liver and secreted into the intestine

� they form the spherical structures (micelles) assisting in absorption

� hydrophobic portion (tails of FA) are located to the inside of the micelle, with heads (hydrophillic portion) to the outside

� they move lipids from the intestinal lumen to the cell surface

� absorption is by diffusion (complete for FA and monoglycerides, less for others)

Factors Affecting Absorption of Lipids

� amount of fat consumed (�fat=�digestion=�absorption)

� age of subject (� age = � digestion)

� emulsifying agents (� digestion = � absorption)

� chain length of FA’s (> 18C = � digestibility)

� degree of saturation of FA (� sat = � digestibility)

� overheating and autooxidation (rancidification (rot) at double bond)

� optimal dietary calcium = optimal FA absorption (high Ca = � absorption)

Lipid Metabolism/Absorption

� short chain FA’s are absorbed and enter the portal vein to the liver

� those FA’s with more than 10 carbons are resynthesized by the liver to triglycerides

� they are then converted into chylomicrons and pass to the lymphatic system

� some FA’s entering the liver are oxidized for energy, others stored

� blood lipids: 45% P-lipids, 35% triglycerides, 15% cholestrol esters, 5% free FA’s

Lipid Digestion/Absorption

Lipid Digestion/Absorption

Characteristics of Fat Storage

� Most of the body’s energy stores are triglycerides

� storage is in adipose, source is dietary

or anabolism (synthesis) from COH or AA carbon skeletons

� remember obesity?

� adipose can remove FA’s from the blood

and enzymes can put them back

Fatty Acid Nomenclature

� Nomenclature reflects location of double bonds

� also used are common names (e.g., oleic, stearic, palmitic)

� linoleic is also known as 18:2 n-6

� this means the FA is 18 carbons in length,

has 2 double bonds, the first of which is on the 6th carbon

� arachidonic = 20:4 n-6

What’s in a Name??

Fatty Acid Nomenclature

Essential Fatty Acids

� Only recently determined as essential (1930)

� body can synthesize cholesterol,

phospholipids

� research: same as AA’s but via addition

(EFA’s added improved growth, NEFA’s didn’t)

� requirement determined by depleting fat

reserves of subject animal: difficult

Essential Fatty Acids (fish) � Most NEAA found in marine food webs

� Essential fatty acids (to date):

– linoleic (18:2 n-6; terrestrials; fish - not really)

– linolenic (18:3 n-3; terrestrials; fish)

– arachidonic (20:4 n-6; marine maybe)

– eicosopentaenoic acid (20:5 n-3, marine)

– docosohexaenoic (22:6 n-3, marine)

� Why? Because elongation beyond 18 carbons is very difficult in marine fish (lack pathways)

� actual EFA requirement is a matter of whether the fish is FW/SW or WW/CW

Essential Fatty Acids (most animals)

� salmonids need n-3 FA’s for membrane flexibility in cold water (why does this work?)

� trout can elongate and desaturate n-3 FA’s

� Linoleic acid (18:2 n-6) is the most essential

� addition of arachidonic is also helpful in deficient diets, but can be synthesized from linoleic (maybe sparing effect)

� EFA’s, like EAA’s, must be dietary

Essential Fatty Acids

LINOLEIC CH3(CH2)4CH=CHCH2CH=CH(CH2)7COOH

18:2 n-6

LINOLENIC CH3CH2CH=CHCH2CH=CHCH2CH=CH(CH2)7COOH

18:3 n-3

EICOSOPENTAENOIC ACID

CH3CH2CH=CHCH2CH=CHCH2CH=CHCH2CH=CHCH2CH=CH(CH2)3COOH

20:5 n-3

DOCOSOHEXAENOIC ACID - YOU CAN DO THIS ONE!

Lipids as Crustacean Energy Sources

� Largely, n-6 FA’s (linoleic) used for energy

� as temperature drops, requirement for

monounsaturated and PUFA’s increases

� change in temperature = change in diet

� cold water species = increased dietary HUFA’s

� maturation animals: increased requirement for 20:4 n-6, 20:5 n-3 and 22:6 n-3 for proper

spawning

Part 2: Carbohydrate Characteristics

From: Lovell; D’Abramo et al.

General Comments

� Carbohydrates often written as “COH”

� much of what we need to know about them, besides their structure, was

covered in “Bioenergetics, Parts 1&2”

� here, we cover structure

Carbohydrate Structure

� Basic chemical structure consists of sugar units

� found as aldehydes or ketones derived

from polyhydric alcohols

� contain: C, H, O

� often shown as aliphatic or linear structures, but exist in nature as ringed

structures

Glucose Structure

O

C-H

H- C-OH

HO-C-H

H-C-OH

H-C-OH

CH2OH

CH2OH

O H

HO

OH

H

H

OH

H

OH

Haworth perspective

Carbohydrate Classification

� Usually by the number of sugar units in the molecule:

– monosaccharides (glucose)

– disaccharides (2 units)

� maltose (2 glucose units)

� sucrose (glucose + fructose)

– polysaccharides (long chain polymers of monosaccharides

– most important polysaccharides to animals are starch and cellulose

Starch and Cellulose CH2OH

O H

OH

H

H

OH

H

CH2OH

O H

OH

H

H

OH

H

O O O

CH2OH

O H

OH

H

H

OH

CH2OH

O

OH

H

H

OH

H

O

O

H

H

O

starch

cellulose

Starch and Cellulose

� Starch contains α-D-glucose linkage

� Cellulose has a β-D-glucose linkage

� we store starch in muscle tissues as

glycogen, peeled off by enzymes when

needed

� cellulose is primary component of plant

tissue, largely indigestible to monogastrics

� must have enzyme, “cellulase”